The present invention relates to an optical semiconductor device and a method of fabricating the same and, more particularly, to an optical semiconductor device having a semi-insulating buried heterostructure wherein an optical integrated circuit consisting of a plurality of waveguides or an optical integrated circuit obtained by adding an electronic device thereto is formed by using a high resistive semi-insulating semiconductor, the optical semiconductor device being represented by an optical cross switch such as a side-light injection type bistable laser or a directional coupler, and a method of fabricating the same.
GaInAsp/InP semiconductor lasers have a basic arrangement in which a layer structure near a light emission region is a double heterostructure consisting of a Ga1-xInxAsyP1-y active layer about 100 nm thick and p- and n-type InP layers (cladding layers). These cladding layers vertically sandwich the active layer and have a larger band gap than that of the active layer.
Due to effective carrier confinement by this double heterostructure, in th active layer of the double heterostructure, it is possible to form excitation carriers at a high density (up to 1018 cm−3) upon energization at a relatively low current density (1 to 10 kA/cm2).
Also, the refractive index of a cladding layer having a large forbidden band is generally smaller than that of an active layer. The double heterostructure uses this refractive index difference to form an optical waveguide in the direction of thickness, confining laser light in the vicinity of the active layer.
A semiconductor laser having this double heterostructure with the above properties can continuously oscillate at room temperature, so the double heterostructure is used as a common basic structure of practical semiconductor lasers.
Furthermore, in practical semiconductor lasers, various stripe structures are used in the horizontal direction parallel to the p-n junction surface to give the lasers the waveguide properties of confining a current or injected carriers into a stripe region in that direction, thereby stabilizing the transverse mode of oscillated laser light.
This stripe active region is sometimes buried in a cladding layer region having a larger forbidden band. The result is a buried heterostructure (BH) in which the double heterostructure is also formed in the horizontal direction.
In this buried heterostructure, carriers are also confined in the horizontal direction, resulting in an increased current injection efficiency.
Furthermore, a two-dimensional optical waveguide is formed by the refractive index difference between the active layer and the cladding region in which the active layer is buried. Consequently, it is possible to obtain a fundamental transverse mode semiconductor laser with a high efficiency and a stable oscillation mode.
One example of the means of forming a buried layer to obtain a buried heterostructure is HVPE (Hydride (Chloride) Vapor Phase Epitaxy) which makes use of a difference from thermal equilibrium by using a nitrogen- or hydrogen-diluted gas mixture of a Group V gas, such as PH3 or PCl3, and a Group III gas of, e.g., a metal halide (InCl) formed by a thermo-chemical reaction with HCl.
MOVPE (Metal Organic Vapor Phase Epitaxy) is also available in which a gasified (diluted) organic metal (primarily of Group III) is crystal-grown together with a Group V gas (PH3) by substrate heating.
Alternatively, the temperature of a solution containing a semiconductor material is decreased by bringing the solution into contact with the substrate surface. Consequently, the liquid phase in the boundary region supersaturates and precipitates (crystal-grows) on the substrate. This method is called LPE (Liquid Phase Epitaxy).
FIGS. 29A and 29B are schematic cross-sectional views showing the arrangements of semiconductor lasers having an SIBH (Semi-Insulating Buried Hetero) structure formed by selectively burying semi-insulating InP using HVPE.
This semiconductor laser is fabricated as follows.
First, an n-type InP cladding layer 2 is crystal-grown on an n-type InP substrate 1 of (001) orientation by using MOCVD or MBE (Molecular Beam Epitaxy). Examples of the dopant for obtaining n-type are Se, Si, and S.
Subsequently, an active layer 3 is formed on top of the structure.
This active layer 3 consists of a guide layer (light confining layer) made from, e.g., undoped or n-type-doped InGaAsP, an active layer formed on the guide layer and made from undoped InGaAsP, and a guide layer formed on the active layer and made from undoped or p-type-doped InGaAs.
Subsequently, a p-type InP overcladding layer 4 is formed on the active layer 3. Examples of the dopant for obtaining a p-type layer are Zn and Be.
A p-type InGaAs or InGaAsP electrode contacting layer 5 is then formed on the overcladding layer 4. This electrode contacting layer 5 is formed to obtain an ohmic contact (and to decrease the contact resistance) with an electrode (to be described later).
An InP layer is sometimes formed on the electrode contacting layer 5 to protect the electrode layer or increase the adhesion of a mask material. However, no such layer is used in this structure.
Subsequently, a stripe pattern (not shown) made from silicon oxide is formed on the electrode contacting layer 5 by photolithography and etching. This stripe pattern is used as a mask to perform etching to a portion below the active layer, forming a stripe etching mesa.
A semi-insulating InP buried layer 6 is then formed to bury the both sides of the etching mesa by using Fe as a dopant. This formation is done by HVPE as described above.
A p-type electrode 7 consisting of an Au—Zn—Ni alloy is formed on the buried layer 6, and an n-type electrode consisting of an Au—Ge—Ni alloy is formed on the lower surface of the substrate 1. Consequently, a semiconductor laser with the structure shown in FIG. 29A is formed.
Note that as the p-type electrode 7, a Ti—Pt—Au alloy can also be Schottky-connected in some instances.
FIG. 29B is a sectional view showing the arrangement of a semiconductor laser formed using an overcladding layer 4a and an electrode contacting layer 5a both increased in area to increase the injection efficiency of carriers into an active layer 3.
In FIG. 29B, reference numeral 9 denotes an n-type InP current blocking layer for suppressing a recombination current with captured electrons resulting from injection of holes from the overcladding layer 4a into a semi-insulating layer 6; and 10, an insulating layer made from silicon oxide or silicon nitride. The rest of the arrangement is similar to that in FIG. 29A.
Note that the current blocking layer 9 is usually formed using the same growth apparatus (e.g., an MOCVD or MOVPE apparatus) as for the p-type overcladding layer 4a. 
On the other hand, as a semiconductor laser in which a buried layer is formed by LPE or MOCVD (MOVPE), semiconductor lasers having a p-n buried structure are available in which, as illustrated in FIGS. 30A and 30B, conductive carriers are confined by forming a p-n junction barrier.
In the structure shown in FIG. 30A, the both sides of a cladding layer 2 and an active layer 3, as an etching mesa, are buried with a p-type InP current blocking layer 11 and an n-type InP current blocking layer 12 by using LPE or MOCVD. Thereafter, a p-type overcladding layer 4a is grown.
FIG. 30B is a sectional view showing the arrangement of a semiconductor laser with a DCPBH (Double Channel Planar Buried Hetero) structure.
In this structure, an etching mesa is not singly formed; that is, an etching mesa consisting of a buffer layer 2 and an active layer 3 is formed by forming trenches.
These trenches are buried with current blocking layers 11a and 12a. 
Note that the same reference numerals as in FIGS. 29A and 29B denote the same portions in FIGS. 30A and 30B.
Also, as with the waveguide lasers described above, vertical resonator type surface emission lasers which vertically emit a laser light from the substrate surface must also have the buried structure to improve the performance.
FIG. 31 is a sectional view showing the major components of a surface emission laser with a GaAs buried structure.
As shown in FIG. 31, this surface emission laser has a cylindrical active region 301 and two reflecting mirrors 302 parallel to each other in the vertical direction. These reflecting mirrors 302 have a DBR (Distributed Bragg Reflector) structure.
The active region 301 is processed into a cylindrical mesa structure 303 by a chemical etching solution and buried with a buried layer 304 formed by LPE.
The buried layer 304 has a pnp structure, and so a current in the transverse direction is blocked.
In FIG. 31, reference numerals 310 and 311 denote cladding layers; 312, a diffused region; 313, an electrode; 314, a buffer layer; 315, a GaAs substrate; and 316, an electrode.
In burying the cylindrical or square active region in this surface emission laser with the above arrangement, the conventional approach is to etch the entire region except for the region to be buried and bury that region using epitaxy such as LPE.
Recently, however, the burying technique using semi-insulating InP has been developed and is beginning to be used as described earlier.
Burying with this semi-insulating InP can improve the performance of a laser, e.g., can increase the modulation rate and decrease the oscillation threshold value.
The application of this technique is expected to improve the performance of surface emission lasers and other surface type optical elements, as well as waveguide lasers.
Unfortunately, the above conventional structures have the following problems.
First, the formation of the SIBH structure using VPE described above is restricted by the structure of a waveguide (etching mesa) to be buried.
As an example, in the formation of a buried layer using MOVPE, the burying of a waveguide extending in the (011) direction is common.
If, however, this is used to bury a waveguide, such as a crossed waveguide or a surface type optical element, having a structure with two or more combinations of different orientations, the reaction site differs from one orientation to another. As a result, around a waveguide in the (0{overscore (1)}1) direction, an abnormal growth such as an overhang readily occurs on the waveguide. This makes flat burying growth impossible.
On the other hand, such an abnormal growth hardly occurs in HVPE since HVPE is a thermal equilibrium process. However, the orientation dependence of the growth rate significantly appears near a regrowth temperature of 650° C. That is, growth in the transverse direction becomes dominant. Accordingly, a slow-growing (001) face is formed in the regrowth process, and this forms a flat surface.
FIGS. 32A and 32B are perspective views showing the arrangement of a crossed waveguide in which a waveguide whose side surfaces are in the (011) direction and a waveguide whose side surfaces are in the (0{overscore (1)}1) direction cross each other.
In FIGS. 32A and 32B, reference numeral 6b denotes a buried layer being grown; and 13, a selective growth mask made from silicon oxide. The rest of the structure is analogous to that shown in FIGS. 29A and 29B.
In this structure, the crystal growth rate of the buried layer 6b in the (011) direction is greatly different from that in the (0{overscore (1)}1) direction; the crystal growth rate around the waveguide in the (0{overscore (1)}1) direction is very high.
The present inventors have also found that the growth rate around the waveguide in the (011) direction, in which the growth rate is originally low, is further lowered in the crossed structure when compared with that in the (011) direction due to mass-transport of a reactant (INCl) and a by-product (HCl) in a surface reaction rate-determining process (kinetic control) under near-equilibrium system.
Consequently, when the both sides of this crossed waveguide are buried with a buried layer, a portion around the waveguide in the (011) direction is not much buried as illustrated in FIG. 32B.
In contrast, the waveguide in the (0{overscore (1)}1) direction is easily covered with the buried layer, making it difficult to perform flat burying growth even with HVPE.
Vertical resonator type surface emission lasers also have the problem of difficulty in flat burying growth, because the side walls of these lasers consist of a large number of orientations.
FIGS. 33A to 33C are views for explaining a method of burying square mesa structures by using HVPE described previously. FIG. 33A is a plan view, FIG. 33B is a sectional view taken along the (011) direction, and FIG. 33C is a sectional view taken along the (0{overscore (1)}1) direction.
No flat surface can be obtained as described above since the growth in the horizontal direction from side walls 335 of a buried mesa structure 333 is faster than the growth in the vertical direction from a bottom surface 336 and since the growth rate changes in accordance with the orientation of the side walls 335.
On the other hand, the formation of a p-n buried structure using LPE has the problems of, e.g., the production of a junction capacitance and the production of a leakage current from the burying interface, and the limitations on the depth of burying for obtaining a pnp layered structure.
Additionally, LPE has a high processing temperature of 700° C. or more, and this degrades the performance of buried elements.